Nanophotonics of ultrathin films and 2D periodic structures: a combined experimental and theoretical study

Photonics is a key enabling technology for many applications ranging from communications to energy and medicine. Its success is largely relying on our capability to appropriately control light in optical devices. To this end, the understanding of light-matter interaction occurring in the devices is a crucial element for finding effective solutions to the challenges posed by the targeted applications. This thesis is devoted to understand light-matter interaction in periodic nanostructures and ultrathin films and create modelling and design tools for functional optical devices, some of them demonstrated experimentally. We start by investigating the needed theoretical methods for describing the interaction of light with surface periodic nanostructures. We carry out a comprehensive study of the transmission, reflection and dispersion properties of 2D periodic arrays and their stacks, including, the study of more complex structures as well, such as, defects in periodic lattices, random arrays of scatterers and multicomponent lattices, and the calculation of the local density of electromagnetic states in the array. We then show how to use the developed theory to design and understand the behaviours of application-specific devices/structures, made of 2D periodic structures and multilayer stack of thin films. A first device demonstrator consists in periodic arrays of nanoholes performated in a gold film covered with Ge2Sb2Te5 (GST), a phase change material layer.We investigate the effect of GST¿s phase transitions on the transmission resonances of these structures. Wavelength shifts as large as 385 nm are demonstrated in configurations with broad resonances. Additionally, excitation of GST with short pulses allows ultrafast tuning of these resonances in the ps regime without producing any phase transition. Finally, tuning of narrower resonances with shifts of 13 nm is also demonstrated. In a second device demonstrator, a perfect absorber, we show how interference effects, occurring in multilayer thin film structures, can be exploited to achieve nearly 100% absorption. Two perfect absorption regimes are identified: the first one broadband and in the visible; the second one resonant and in the near infrared (NIR) region of the wavelengths. We show that the proposed method enables conceptually simple devices that are easy to fabricate. Moreover, we show that GST constitutes an essential layer for a new class of optical absorbers that can be dynamically tuned. In contrast, previous structures required cumbersome fabrication steps and were not dynamically tunable. In a third device demonstrator, a structure with multilayer thin films is used to design and fabricate an anti-reflective, highly transparent electrode, with world-record low sheet electrical resistance and high optical transmission. In summary, the thesis capitalizes on modelling tools for light-matter interaction at the nano-scale, which are adapted to a general class of device structures and allow us to design optical surfaces based on thin films and nano-structuring with unprecedented performance. This is demonstrated through the design and experimental realization of resonant optical filters with very large tunability, perfect absorbers with very high dynamic range and transparent electrodes with record electro-optical performance.La fotònica és una tecnologia que permetrà implementar noves tecnologies en àrees tan diverses com les comunicacions, l'energia o la medicina. El seu èxit dependrà en gran mesura de la capacitat de controlar la llum en els dispositius òptics. En aquest sentit, entendre com la llum i la matèria interaccionen en aquests dispositius és un dels requisits principals a l'hora de trobar solucions efectives als reptes que ofereixen les diferents àrees d'aplicació de la fotònica. Aquesta tesi té com a objectiu entendre les interaccions entre llum i matèria en estructures periòdiques i capes ultra-primes així com crear eines de disseny i modelat de dispositius òptics, alguns dels quals són també demostrats experimentalment. A la primera part de la tesi s'investiga la teoria necessària per descriure la interacció de la llum en superfícies periòdiques nano-estructurades. Això inclou un estudi detallat de la transmissió, reflexió i dispersió d'estructures periòdiques en 2D o combinacions d¿elles, així com també l'estudi d'estructures més complexes, com ara defectes, estructures aleatòries, i finalment el càlcul de la densitat local d'estats electromagnètics en aquestes estructures. A la segona part de la tesi s'aplica aquesta teoria per dissenyar i entendre el comportament de dispositiu fotònics basats en aquestes estructures 2D per a aplicacions específiques. El primer dispositiu que es demostra consisteix en una estructura periòdica de nano-forats en una capa d'or coberta amb Ge2Sb2Te5 (GST), un material de canvi de fase. S'investiga l'efecte que té un canvi de fase en la capa de GST en les ressonàncies de transmissió d'aquestes estructures i es demostren canvis en la longitud d'ona de ressonància de fins a 385 nm en el cas de ressonàncies amples. A més a més també es demostra com excitant la capa de GST amb polsos ultra-curts aquestes ressonàncies també es poden modificar en una escala de temps de ps sense la necessitat de tenir un canvi de fase. Per últim també es demostren canvis en la longitud d'ona de ressonàncies de fins a 13 nm en dispositius amb ressonàncies estretes. En el segon dispositiu es demostra com els efectes d'interferència que tenen lloc en estructures compostes per vàries capes primes poden ser explotats per tal d'obtenir una absorció de gairebé el 100%. En particular es demostren dos règims d'absorció completa: banda ampla en el visible i absorció ressonant en l'infraroig. Aquest mètode permet fabricar dispositius de manera fàcil. A més a més es demostra com el GST permet crear una nova classe de dispositius amb aborció completa que poden ser sintonitzats dinàmicament, en contrast amb la majoria d'estructures proposades fins al dia d'avui. En la tercera aplicació es dissenya i demostra experimentalment una estructura de vàries capes per a ser usada com a elèctrode transparent amb propietats d'antireflexió, i amb una resistència molt baixa i alta transmissió òptica. En resum, aquesta tesi descriu eines per modelar la interacció entre llum i matèria en l'escala dels nanòmetres per una classe general d'estructures que després són usades per dissenyar superfícies òptiques basades en capes primes i nano-estructuració. En particular això es demostra amb el disseny i realització experimental de filtres òptics ressonants, dispositius amb absorció completa i gran rang dinàmic així com elèctrodes transparents amb unes grans propietats electró-òptiques

An antireflection transparent conductor with ultralow optical loss (o2 %) and electrical resistance (o6O 2)

Transparent conductors are essential in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and solar cells. Here we demonstrate a transparent conductor with optical loss of B1.6%, that is, even lower than that of single-layer graphene (2.3%), and transmission higher than 98% over the visible wavelength range. This was possible by an optimized antireflection design consisting in applying Al-doped ZnO and TiO2 layers with precise thicknesses to a highly conductive Ag ultrathin film. The proposed multilayer structure also possesses a low electrical resistance (5.75O 2), a figure of merit four times larger than that of indium tin oxide, the most widely used transparent conductor today, and, contrary to it, is mechanically flexible and room temperature deposited. To assess the application potentials, transparent shielding of radiofrequency and microwave interference signals with B30 dB attenuation up to 18 GHz was achieved.Peer ReviewedPostprint (author's final draft

Ultrafast Momentum-Resolved Probing of Plasmon Thermal Dynamics with Free Electrons

Current advances in ultrafast electron microscopy make it possible to combine optical pumping of a nanostructure and electron beam probing with sub{\aa}ngstrom and femtosecond spatiotemporal resolution. We present a theory predicting that this technique can reveal a rich out-of-equilibrium dynamics of plasmon excitations in graphene and graphite samples. In a disruptive departure from the traditional probing of nanoscale excitations based on the identification of spectral features in the transmitted electrons, we show that measurement of angle-resolved, energy-integrated inelastic electron scattering can trace the temporal evolution of plasmons in these structures and provide momentum-resolved mode identification, thus avoiding the need for highly-monochromatic electron beams and the use of electron spectrometers. This previously unexplored approach to study the ultrafast dynamics of optical excitations can be of interest to understand and manipulate polaritons in 2D semiconductors and other materials exhibiting a strong thermo-optical response.Comment: 24 pages, 13 figures, 101 reference

Plasmonic Enhancement of Second Harmonic Generation in Weyl Semimetal TaAs

In this work a hybrid nanoplasmonic-Weyl Semimetal (WSM) structure is realized for the first time utilizing silver nanopatch antennas and WSM Tantalum Arsenide (TaAs). The studied hybrid WSM-nanoplasmonic structure demonstrated a substantial, over x4.5 enhancement of the second harmonic generation (SHG) process compared to a bare TaAs film. To realize the hybrid structure while preserving TaAs properties, a scalable, non-destructive manufacturing approach was developed that involves the fabrication of TaAs flakes from single crystalline TaAs, overgrowth of a silicon nitride overlayer, and drop-casting of silver nanopatch antennas. The strong polarization response of both the bare flakes, along with the hybrid-nanoplasmonic cavities demonstrates that this approach uniquely preserves the TaAs crystal structure and its optical response while providing significant enhancement of the nonlinear properties. The developed method allows leveraging the capabilities of plasmonics to control and enhance light-matter interactions at the nanometer scale to access and engineer WSM response. This work is the first step towards high-performance nanophotonic devices utilizing WSM topological properties

Ultrathin Transparent Conductive Polyimide Foil Embedding Silver Nanowires

Metallic nanowires are among the most promising transparent conductor (TC) alternatives to widely used indium tin oxide (ITO) because of their excellent trade-off between electrical and optical properties, together with their mechanical flexibility. However, they tend to suffer from relatively large surface roughness, instability against oxidation, and poor adhesion to the substrate. Embedding in a suitable material can overcome these shortcomings. Here we propose and demonstrate a new TC comprising silver nanowires (AgNWs) in an ultrathin polyimide foil that presents an optical transmission in the visible larger than ITO (>90%), while maintaining similar electrical sheet resistance (15 ohm/sq). The polyimide protects the Ag against environmental agents such as oxygen and water and, thanks to its deformability and very small thickness (5 μm), provides an ideal mechanical support to the NW’s network, in this way ensuring extreme flexibility (bending radius as small as at least 1 mm) and straightforwardly removing any adhesion issue. The initial AgNWs’ roughness is also reduced by a factor of about 15, reaching RMS values as low as 2.4 nm, suitable for the majority of applications. All these properties together with the simple fabrication technique based on all-solution processing put the developed TC in a competitive position as a lightweight, mechanically flexible and inexpensive substrate for consumer electronic and optoelectronic devices